A contra-rotating open rotor (CROR) propulsion system for an aircraft typically includes two engines mounted on either side of an aft portion of the aircraft fuselage. Each engine powers contra-rotating rotors that each includes a plurality of blades. Typically small turbine engines are utilized to drive a contra-rotating gearbox that provides the opposite rotation of the open rotors. Propulsive efficiency is provided by the large bypass flow at relatively low velocities. The large bypass volumes along with the lower velocities also provide favorable noise properties.
Although smaller turbine engines are often utilized, the radial cross-section of the engine reduces the effective area of the open rotors and thereby can reduce overall propulsive efficiency. The turbine engines' radial cross-section also causes increased resistance thus affecting adversely the aircraft's overall efficiency. Moreover, the support structure required for supporting the weight of the gas turbine engine away from the aircraft can limit the overall size of the contra-rotating open rotors and thereby overall propulsive efficiency.
Accordingly, improvements in contra-rotating propulsion systems are part of continued efforts by engine and aircraft manufacturers to improve engine performance and propulsive efficiencies.